libweston: per-output z-ordered paint node list

This patch creates a per-output paint node list in the same z-order as
the global view_list in weston_compositor.

The next step is to switch output repaints and backends to use the
z-order list instead of view_list.

Having a per-output paint node list for repaints allows including only
those paint nodes that actually contribute to the output image, so that
completely occluded and out-of-screen views can be ignored in libweston
core already.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

include/libweston/libweston.h

@@ -291,6 +291,12 @@ struct weston_output {
 	struct wl_list animation_list;
 	int32_t x, y, width, height;
 
+	/** List of paint nodes in z-order, from top to bottom, maybe pruned
+	 *
+	 *  struct weston_paint_node::z_order_link
+	 */
+	struct wl_list paint_node_z_order_list;
+
 	/** Output area in global coordinates, simple rect */
 	pixman_region32_t region;